1. Field of the Invention
This invention relates in general to dual spin valve heads for magnetic storage systems, and more particularly to a method and apparatus for enhanced dual spin valve giant magnetoresistance (GMR) effects using two spin valves with the second spin valve having a self-pinned composite layer.
2. Description of Related Art
The size and complexity of operating systems, user applications and data files continues to increase. As a result, the importance of magnetic storage systems is also increasing. To increase storage capacity, the performance of magnetic heads is one focus of much research. Magnetic heads are used for writing and reading data on magnetic recording media. Anisotropic Magnetoresistive (AMR) technology has been the primary, high performance read technology. An AMR head employs a special conductive material that changes its resistance in the presence of a magnetic field. As the head passes over the surface of the disk, this material changes resistance as the magnetic fields change corresponding to the stored patterns on the disk. A sensor is used to detect these changes in resistance, which allows the bits on the platter to be read.
However, even AMR heads have a limit in terms of how much areal density they can handle. Successive generations of AMR heads were reduced in size to allow still greater areal density. Sometimes these more advanced designs were dubbed MRX for Extended Magnetoresistive heads. However, giant magnetoresistive (GMR) heads is the current focus in the logical progression of the storage industry's endless quest for a way to increase areal density while reducing the price per megabyte.
A typical GMR head design consists of a thin film inductive write element and a read element. The read element consists of an GMR sensor between two magnetic shields. The magnetic shields greatly reduce unwanted magnetic fields coming from the disk; the GMR sensor essentially “sees” only the magnetic field from the recorded data bit to be read. In a merged head the second magnetic shield also functions as one pole of the inductive write head. The advantage of separate read and write elements is both elements can be individually optimized. A merged head has additional advantages. This head is less expensive to produce, because it requires fewer process steps; and, it performs better in a drive, because the distance between the read and write elements is less.
During operation, the inductive write head records bits of information by magnetizing tiny regions along concentric tracks on a disk. During reading, the presence of a magnetic transition or flux reversal between bits causes the magnetic orientation in the GMR sensor to change. This in turn, causes the resistance of this sensor to change. The sensor's output voltage or signal is the product of this resistance change and the read bias current. This signal is amplified by low-noise electronics and sent to the HDD's data detection electronics.
GMR sensors are composed of multiple thin films. GMR sensors have a sensing layer, which responds to external magnetic fields. GMR sensors include two magnetic layers separated by a spacer layer chosen to ensure that the coupling between magnetic layers was weak, unlike previously made structures. The magnetic orientation of one of the magnetic layers is also “pinned” in one direction by adding a fourth strong anti-ferromagnetic layer. In this arrangement, the anti-ferromagnetic layer biases one of the magnetic layers. Alternatively, a self-pinned magnetic layer may be used. A self-pinned layer has a magnetic moment which is pinned parallel to the magnetic moment by sense current fields from the conductive layers.
The key structure is a spacer layer of a non-magnetic metal between two magnetic metals. Magnetic materials tend to align themselves in the same direction. So if the spacer layer is thin enough, changing the orientation of one of the magnetic layers can cause the next one to align itself in the same direction. During operation, the magnetic alignment of the magnetic layers swing back and forth from being aligned in the same magnetic direction (parallel alignment) to being aligned in opposite magnetic directions (anti-parallel alignment). The overall resistance is relatively low when the layers are in parallel alignment and relatively high when in anti-parallel alignment. When a weak magnetic field, such as that from a bit on a hard disk, passes beneath such a structure, the magnetic orientation of the unpinned magnetic layer rotates relative to that of the pinned layer, generating a significant change in electrical resistance due to the GMR effect.
A dual spin valve arrangement may also be used. With this arrangement, the magnetoresistive coefficient is increased due to the spin valve effect on each side of the free layer. The dual spin valve sensor typically includes a five layer GMR film. The five layer GMR film includes three ferromagnetic layers separated by two thin conductive metallic layers. The two outer ferromagnetic layers are generally exchange coupled. The pinned layer may have its magnetization pinned by exchange coupling with an anti-ferromagnetic (e.g., NiO or FeMn) layer. Alternatively, a sense current may be used as the means for pinning the pinned layer magnetization as opposed to the use of the conventional anti-ferromagnetic layer.
However, each arrangement has its problems. Sensors formed with two exchange coupled spin valves are too thick due to the presence of thick anti-ferromagnetic pinning layers. This thickness is a disadvantage that severely limits the sensitivity of these sensors to minute changes in the magnetic flux. Still, this dual exchange coupled spin valve sensor has at least one advantage, one being that the magnetic moment pinned through exchange coupling is strongly pinned thereby causing the free layer in the exchange coupled spin valve system to exhibit stable biasing. The result is a reliable, albeit, poorly sensitive, sensor.
The dual spin valve arrangement wherein the dual spin valve sensor is formed with two self pinned spin valves do not require an adjacent anti-ferromagnetic layer to pin the magnetic moment. Advantageously, this type of dual spin valve sensor can be fabricated thinner than the dual exchange coupled spin valve which would lead to a more sensitive sensor. However, the dual self pinned spin valve sensor lacks the stable biasing of the dual exchange coupled spin valve sensor. The result is a sensitive, but unreliable and unstable sensor.
It can be seen that there is a need for a method and apparatus for providing enhanced giant magnetoresistance (GMR) effects to provide increased sensitivity to minute changes in resistance in response to magnetic flux interactions.